A clinical psychologist said on Friday it's normal for people who witnessed traumatic events to have negative emotions, and even suffer from physical symptoms, and that it's important for them to get the help they need.

The Hong Kong Red Cross said it had received at least 90 calls and messages seeking help as of 10am Friday after setting up a dedicated counselling hotline following an incident at the MIRROR concert on Thursday night. A falling video panel left two dancers injured and concertgoers in shock.

"Some of the callers – they were there witnessing the happening of the whole accident – even asked us whether or not it was a real thing that happened," said Eliza Cheung, a clinical psychologist at the organisation.

She told RTHK that people also reported shock, anger, numbness and even physical reactions such as nausea, stomachache and headache. "These are normal reactions and we should not pathologise all these normal reactions."

Cheung advised against watching footage of the incident repeatedly to avoid triggering further impact.

She added that professional help may be needed if the symptoms linger for a month.

"There is a possibility for them to develop what we call the acute stress disorder that could happen at the very first phase after the crisis... they may also develop what we usually heard of as post-traumatic stress disorder," she said.

Cheung suggested that people can share their feelings with someone they trust, noting that in times of crisis, the support of family and friends is effective to help them cope.

As for those being approached for help, Cheung highlighted the importance of being sensitive when trying to lend an ear.

"If they don't feel like talking about what they went through or what they witnessed, we don't just dig into the details of the incident because of our curiosity, because by doing that we may also re-traumatise them by asking them to recall the details of the incident all over again."